In response to the previous thread about what browser you use, I figured I'd find out what add-ons you guys are using for FireFox (since it's an overwhelming majority)
As a web developer, my most useful one is Firebug.
I also use Foxmarks Bookmark Synchronizer. I use FireFox on about 3 computers, and it's so nice to bookmark it once, and it appears on all 3.
I also use ColorfulTabs and a few other ones.
If you're a die-hard IE user or Safari user - Some of the add-ons will definitely change your mind about FireFox